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This topic comprises 2 pages: 1 2
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Author
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Topic: Luminance meter
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 07-16-2011 08:10 AM
Seriously...what is the value of a cheap meter whose reading is in question? I understand that the cost of proper meters is out of the reach of most individuals but that is truly besides the point. A device that outputs a number does not necessarily make it of value. Maybe it makes you feel better because the gadget in your hand says 14fL or 5fL or whatever but it doesn't mean that you are really getting that value.
And just where are you pointing that meter? Light is not a point-n-shoot situation to set levels. Even more absurd is the thought of using an incident meter to figure out the actual reflected light. It presumes that the screen's specular response remains consistent as it ages. It does not (neither in color or reflectance). The odds you hold the meter in the exact same spot each time also brings a flaw into the equation. If your cold mirror, or reflector or lamp gets changed, moved...etc your control point is moved and makes the reading completely invalid (no two lamps are made the same...hence they need to be adjusted on each installation).
What would likely work better as a "cheap" meter is to, right after a technician with a CALIBRATED light meter sets up your system...get a camera with adjustable shutter and aperture and a tripod. Clean the port where the camera will shoot through. First project a crosshair target (there should be one in all DLP projectors) and point the camera's center to that point, then switch to white light and adjust the shutter speed and f-stop for a "perfect exposure".
At least with that set up, you have a repeatable set up...your calibration will vary a bit as the camera's sensor ages (or if you damage the camera)...you will likely find that if you check from month to month, it remains pretty close. Don't bother using the 3D glasses (or 3D apparatus in front of the lens/light) because all you are looking for is a change in light level. Whatever the magic setting is for 2D or 3D for your camera...that is what you are looking for.
Note, you can obviously use your camera in the theatre, instead of the booth (especially since booths are becoming less accessible or non-existant but try to make sure you pick the same seat each time, if your lens is a zoom, make sure it is set to the exact same position each time (best to have it at an extreme to it is repeatable).
Cheap meters are not the way though...it may give one comfort to think they have measured a desired light reading but it is just comfort, not necessarily a real reading. It gives one a false-sense of accuracy.
What surprises me is this sudden concern about light levels...it is no more critical for digital than it was for film. Yet now, folks seem like they want to have the means to measure the light. I love the idea that people are concerned enough to try and put out the best quality show and don't want to discourage that but using devices that do not necessarily tell a true picture (no pun) are not the way.
Even the good meters need regular calibration if they are be of any value (not just light meter, most any meter). We spend a small fortune annually to keep our test equipment in proper calibration...both light and sound meters. The net result, I can take three different light meters (from different manufacturers even) and yield identical results...it gives credibility to the readings. I wonder how many people that set up the color on these projectors actually also VERIFY that when the correction is applied that the color is correct.
-Steve
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